The adjustment of the shape or curvature of the lens inside the eye so as to attain visual acuity (clear, sharp focus of sight). For example, when the eye focuses on an object in the distance the curvature decreases, whereas whereas when the eye focuses on an object at near the lens become more curved.

ACUITY: Sharpness or clarity of vision

AMBLYOPIA ("lazy eye"): a visual defect that affects approximately 2 or 3 out of every 100 children in the United States. Amblyopia involves lowered visual acuity (clarity) and/or poor muscle control in one eye. The result is often a loss of stereoscopic vision and binocular depth perception. Vision therapy can benefit this condition, but early detection is very important. For many years, it was thought that amblyopia (lazy eye) was only amenable to treatment during the "critical period". This is the period up to age seven or eight years. Current research has conclusively demonstrated that effective treatment can take place at any age, but the length of the treatment period increases dramatically the longer the condition has existed prior to treatment. Research has also demonstrated that patients with amblyopia are more likely to sustain injuries resulting in the loss of their good eye than individuals with two good eyes. There are many reasons that early childhood eye examinations are essential. See the Directory of Vision Care Providers for a free referral to an eye doctor who offers comprehensive eye examinations.

DYSLEXIA: (dyslexic). Margaret Livingstone, Department of Neurobiology, Harvard Medical School and the Dyslexia Research Laboratory, Beth Israel Hospital in Boston defined dyslexia as follows: "Developmental dyslexia is the selective impairment of reading skills despite normal intelligence, sensory acuity, and instruction. Several perceptual studies have suggested that dyslexis subjects process visual information more slowly than normal subjects. Such visual abnormalities were reported to be found in more than 75% of the reading-disabled children tested." Therefore, it is important to rule out problems with sensory acuity (including visual acuity and visual processing) before labelling an individual as truly dyslexic.

BEHAVIORAL OPTOMETRY:
an international branch of optometry that specializes in the rehabilitation of functional vision through the practice of vision therapy. Behavioral optometrists sometimes consider how environmental, nutritional and/or behavioral factors affect visual health.

BINOCULAR VISION:
vision as a result of both eyes working as a team; when both eyes work together smoothly, accurately, equally and simultaneously.

STEREOSCOPIC VISION:
(stereopsis or stereoscopic vision): a byproduct of good binocular vision; vision wherein the separate images from two eyes are successfully combined into one three-dimensional image in the brain. For the complete scoop, visit an illustrated page on stereo vision!

BINOCULAR DEPTH PERCEPTION:
a result of successful stereo vision; the ability to visually perceive three dimensional space; the ability to visually judge relative distances between objects; a visual skill that aids accurate movement in three-dimensional space.

BINOCULAR VISION IMPAIRMENT:
A visual defect in which the two eyes fail to work together as a coordinated team resulting in a partial or total loss of binocular depth perception and stereoscopic vision. At least 12% of the population has some type of binocular vision disability. Amblyopia and strabismus are the most commonly known types of binocular vision disabilities. To learn more, see What is a binocular vision impairment?

EYE-HAND COORDINATION: a type of "fine motor coordination" -- the ability of the eye and the hand to coordinate during a specific action, such as handwriting or copying from the chalkboard

STRABISMUS: ("crossed eye", "wall eye", "wandering eye", esotropia, exotropia, hyperphoria) affects approximately 4 out of every 100 children in the United States. It is a visual defect in which the two eyes point in different directions. One eye may turn either in, out, up, or down while the other eye aims straight ahead. Due to this condition, both eyes do not always aim simultaneously at the same object. This results in a partial or total loss of stereo vision and binocular depth perception. The eye turns may be visible at all times or may come and go. In some cases, the eye misalignments are not obvious to the untrained observer. A consultation with an optometrist who offers supervised vision therapy is recommended with this condition. See the Directory of Vision Care Providers.

OPTOMETRIST OR PEDIATRIC OPTOMETRIST:
a doctor of optometry who diagnoses and treats visual health problems as dictated by state law. In most states, optometrists are licensed to examine visual health, prescribe glasses and contact lenses, fit special devices for vision-impaired individuals, treat eye diseases, prescribe drugs and perform vision therapy.

OPHTHALMOLOGIST OR PEDIATRIC OPHTHALMOLOGIST:
A doctor of medicine (M.D.) specializing in diseases of the eye and surgery. To learn more about the differences between the practices of ophthalmologists and optometrists, visit our web page on Choosing an Eye Doctor.

ORTHOPTIC THERAPY:
a restricted form of vision therapy which teaches only the abilities of eye teaming and visual acuity and does not treat other visual dysfunctions that may be addressed in other types of vision therapy. This therapy first became popular in Europe in the 1900s. David Wells, M.D., an ophthalmologist at Boston University, is credited with introducing orthoptics to the U.S. in 1912. Still practiced by optometrists, ophthalmologists and orthoptic therapists.

PHORIA: the tendency for a horizontal or vertical misalignment of the two eyes

SCANNING: the ability of the eyes to look at or between letters or words. Children without this skill tend to guess at words and have a hard time learning the alphabet.

SACCADIC EYE MOVEMENTS: the motion where the eye jumps from one object to another; eye movements which occur between words or groups of words, each fixation averaging one-fourth of a second.

SNELLEN CHART or SNELLEN EYE CHART: This is the standard eye chart used to measure visual acuity at distance only (typically a distance of 20 feet).

SUPPRESSION: the suspension by the brain of the visual information that is coming in through one eye; the eye is healthy and receives information, but brain ignores the information in order to compensate for misaligned eyes and resulting misaligned images or other problems with stereoscopic vision.

VISION:
The act of perceiving visual information with the eyes, mind, and body.